1. Field of the Invention
This invention relates to a resistance change memory device for storing a rewritable resistance value as data in a non-volatile manner, specifically relates to a reference cell used for detecting a selected cell's current.
2. Description of the Related Art
It has been proposed such a resistance change memory (ReRAM) that stores a resistance value as data, which is reversibly exchanged by applying voltage, current or heat, and it is noticed for succeeding to the conventional NAND-type flash memory. This resistance change memory is suitable for shrinking the cell size, and for constituting a cross-point cell array. In addition, it is easy to stack cell arrays.
Specifically, a unipolar type of ReRAM cell has such a feature that the high resistance state and low resistance state are reversibly settable by controlling the applied voltage and applying time thereof. For example, refer to Y. Hosoi et al, “High Speed Unipolar Switching Resistance RAM(RRAM) Technology” IEEE International Electron Devices Meeting 2006, Technical Digest, P. 793-796.
However, in accordance with the variation of the manufacturing process, the stable value of the resistance state of the recording layer may be changed for every memory chip based on a slight composition variation, and the wiring resistance also varies. In addition, in a memory chip having a three-dimensionally stacked cell array, the wiring resistance in the sense amplifier is largely changed dependent on the position in the cell array, so that it becomes difficult to judge a cell's resistance state as a constant data state in the whole memory chip.
Therefore, it is in need of setting data and reading data in a cell in consideration of the variation of the cell state in accordance with the manufacturing place and time, and the position in each memory chip. Usually, cell data read is performed in such a way as to compare a selected cell's current with a reference current of a reference cell with a sense amplifier. This is identical in both of the normal data read and the write-verify read.